There is a well-known procedure for reducing NO.sub.x emissions from furnaces known as reburn and by various other terms. In this procedure a fuel, usually natural gas or sometimes pulverized coal, is introduced into the furnace above the primary combustion zone. Usually more than enough fuel is added to react with all of the oxygen remaining in the original combustion products. A reducing zone, or a zone with an excess of fuel, is formed. In this reducing zone the NO reacts with the excess fuel to form N.sub.2, NH.sub.3, HCN, and other reduced nitrogen. Then more completion air is added to combust the remainder of the reburn fuel. At this point the NH.sub.3, HCN, and other reduced forms are oxidized to N.sub.2 and NO. At this step and through out the mixing process there is also a direct reaction between NO and NH.sub.3 to form N.sub.2. In each step, part of the fixed nitrogen (originally NO) was converted to N.sub.2. This is the goal of the reburn process.
Sometimes a modified reburn process is used in which the reburn fuel is not added in sufficient qualities to consume all of the oxygen remaining in the gas after the initial combustion. In such a process it is necessary that large volumes become reducing while parallel volumes remain oxidizing. In the reducing volumes N.sub.2, NH.sub.3, and HCN are formed. Then the reducing and oxidizing gases mix together and the remainder of the fuel is consumed. At this point the reduced nitrogen species are oxidized to N.sub.2 and NO. Again there is direct reduction to N.sub.2 by the reaction between NH.sub.3 and NO. This modified or fuel lean process is simplified by the lack of need for completion air and by requiring less reburn fuel and also less reburn fuel mixing with associated furnace volume requirements.
Another version of the reburn process is to inject a gas as the reburn fuel into the flue gas stream. This process usually requires natural gas as the reburn fuel. Natural gas is expensive. Although natural gas alone has been injected as a reburn fuel, often it is necessary to use a carrier gas to assure adequate penetration of the natural gas into the furnace. If the reburn natural gas is 5% of the fuel and the fuel is only 10% of the air flow, the reburn gas is perhaps only 0.5% of the flue gas flow. The combustion products being quite hot may have a volume as high as 1000 times the reburn fuel. Utility boiler furnaces have horizontal dimensions of 50 feet and greater. Penetration and mixing is a great problem.
In those processes where a carrier gas is used the carrier gas may be steam, air, or combustion products. The steam is expensive. The use of air or recycled combustion products requires expensive duct work. Often there is no place for the duct work. The boiler face is simply too crowded with necessary equipment to allow the duct work to be installed. When a carrier gas is used large penetrations through the furnace walls are needed and this requires bending water wall tubes. If flue gas is used as the carrier, extensive duct work is usually required because the flue gas must be returned from a remote part of the boiler. Fans are needed for flue gas and often for air. The air has oxygen in it, which requires more reburn fuel before the gas stream can be made reducing.
Some operators have tried coal as a reburn fuel. The burnout times are longer. This requires that both the fuel and the burn out air be added sooner. As a result much of the reaction occurs at higher temperatures, which results in more NO.sub.x emissions. The use of coal requires that there be additional pipes to carry primary air and pulverized fuel from the mills usually at ground level to the height where the reburn fuel is injected. It may even require an additional pulverizer.
We have experimented with the use of coal water slurry and have used it in the manner of fuel lean gas reburn. That is, not enough fuel was injected into the upper furnace to make the total flow fuel rich and no burn out air was added. The coal water slurry was used in a Fuel Lean Reburn System. This method is described in U.S. Pat. No. 5,746,144, issued May 5, 1998 to by B. P. Breen, J. E. Gabrielson and J. Cavello for "Method And Apparatus For NO.sub.x Reduction By Upper Furnace Injection Of Coal Water Slurry",
While all of these reburn methods have been successful in reducing NO.sub.x emissions, the industry has been slow to adopt them. For many the associated costs and installation problems discussed above when considered with the expected level of NO.sub.x emissions reduction has not been perceived to be worth the investment. Consequently, there continues to be a need for a reburn method which provides significant NO.sub.x emissions reduction without requiring extensive duct work or significant modifications to the wall of the furnace.